Frequently Asked Questions about Photovoltaics

How much electricity do PV systems generate?
The capacity of a PV system is stated in terms of the number of watts, kilowatts, or megawatts it can produce in standard sunlight conditions. For example, in the city of Chicago, Illinois, U.S., GloriaSpire has installed solar modules for which each kilowatt (kW) of PV capacity produces about 1,200 - 1,400 kilowatt-hours per year, depending on the angle at which the panels are mounted.

What do the terms watts, kilowatts and kilowatt-hours represent?
A watt is a unit of power. A kilowatt is one thousand watts and a megawatt is one million watts. A watt-hour is a unit of energy, or power multiplied by time. A kilowatt-hour is one thousand watt-hours and a megawatt is one million watt-hours. The electricity you purchase from the utility is measured in kilowatt-hours (kWh). Most single-family homes in the U.S. use 20 kWh or more per day. A refrigerator uses 1.2 kWh per day and a computer uses 2.4 kWh per 8-hour workday.

What percentage of my electricity will my PV system generate?
That depends on how much electricity you use in a year. You can calculate this by adding the kilowatt-hours (kWh) found on your monthly electricity bills during the past year. Or, you can call your electricity provider for a history of your electricity consumption. Once you have that number, divide the expected annual output of the PV system by your annual consumption. In most cases, PV systems are designed to be supplementary sources of power. To decrease your electricity use, consider installing compact florescent light bulbs and replacing older appliances with ENERGY STAR(R)-rated appliances.

What's the difference between AC and DC electricity?
Alternating current (AC) electricity is the type of electricity used by lights, electrical appliances, and most of machines around the world. The voltage of this electricity is constantly alternating. PV systems, however, produce direct current (DC) electricity, which has a constant voltage. This electricity has to be converted to AC by electronic equipment called an inverter in order for it to be used by your electrical devices. The wires and inverter causes a slight loss of energy, so the AC output is about 80% of the DC electricity generated.

Do I have to select certain appliances to be powered by the PV system?
No, the solar electricity is fed through your main electrical distribution panel and can be used by any lights and appliances just like electricity from your local utility.

How much roof space do I need for a PV system?
A typical PV system requires about 100 square feet for every kilowatt installed on a pitched roof, and up to 200 square feet per kilowatt on a flat roof. PV systems should be installed facing south for maximum electricity production.

Will a PV system cause my roof to leak?
Most flat roof installations do not require penetrations, and therefore do not increase the chance of leaks. A PV system can actually protect a flat roof beneath it from ultraviolet rays and leaks. A pitched roof installation will require penetrations, but each hole is protected by standard roof flashing and the chance of a leak is not increased.

Are PV systems expensive?
Systems of any kind that safely produce electricity are expensive. When you buy a PV system, you are effectively buying electricity for the next 20 years or more up-front, avoiding future electricity price volatility. Solar electricity is also a premium grade of electricity because it is generated without giving off any pollutants. This makes it more valuable and more expensive than standard utility electricity.